Looks like it spent some time in a not so friendly environment. I'd say its either corrosion or toning
I also think it is just surface discoloration, but if you look at the reeded edge, do you see a layer ?
I've seen a few coins that look like that and your discolored quarters. That is all natural toning and dirt. Very common in coins found in pocket change.
Environmental damage. The weight is too high to be a missing layer and way too high to be two missing layers. Plus the strike is way too good to be two missing layers.